Flux-biasing superconducting quantum processors

ABSTRACT

A flux-biasing device includes a set of magnetic flux generating members. A first magnetic flux generating member is configured to magnetically interact with a first qubit from a set of qubits of a quantum processor such that a first magnetic flux of the first member causes a first change in a first resonance frequency of the first qubit by a first frequency shift value. Each non-corresponding magnetic flux generating member of the set is well separated from qubits corresponding to other magnetic flux generating members of the set such that qubits corresponding to other members exhibit less than a threshold value of resonance frequency shift as a result of a magnetic flux of a non-corresponding member.

TECHNICAL FIELD

The present invention relates generally to a device, a fabricationmethod, and fabrication system for adjusting the resonance frequenciesin quantum processors. More particularly, the present invention relatesto a device, method, and system for flux biasing in superconductingquantum processors.

BACKGROUND

Hereinafter, a “Q” prefix in a word or phrase is indicative of areference of that word or phrase in a quantum computing context unlessexpressly distinguished where used.

Molecules and subatomic particles follow the laws of quantum mechanics,a branch of physics that explores how the physical world works at themost fundamental levels. At this level, particles behave in strangeways, taking on more than one state at the same time, and interactingwith other particles that are very far away. Quantum computing harnessesthese quantum phenomena to process information.

The computers we use today are known as classical computers (alsoreferred to herein as “conventional” computers or conventional nodes, or“CN”). A conventional computer uses a conventional processor fabricatedusing semiconductor materials and technology, a semiconductor memory,and a magnetic or solid-state storage device, in what is known as a VonNeumann architecture. Particularly, the processors in conventionalcomputers are binary processors, i.e., operating on binary datarepresented in 1 and 0.

A quantum processor (Q-processor) uses the odd nature of entangled qubitdevices (compactly referred to herein as “qubit,” plural “qubits”) toperform computational tasks. In the particular realms where quantummechanics operates, particles of matter can exist in multiplestates—such as an “on” state, an “off” state, and both “on” and “off”states simultaneously. Where binary computing using semiconductorprocessors is limited to using just the on and off states (equivalent to1 and 0 in binary code), a quantum processor harnesses these quantumstates of matter to output signals that are usable in data computing.

Conventional computers encode information in bits. Each bit can take thevalue of 1 or 0. These 1s and 0s act as on/off switches that ultimatelydrive computer functions. Quantum computers, on the other hand, arebased on qubits, which operate according to two key principles ofquantum physics: superposition and entanglement. Superposition meansthat each qubit can represent both a 1 and a 0 at the same time.Entanglement means that qubits in a superposition can be correlated witheach other in a non-classical way; that is, the state of one (whether itis a 1 or a 0 or both) can depend on the state of another, and thatthere is more information that can be ascertained about the two qubitswhen they are entangled than when they are treated individually.

Using these two principles, qubits operate as more sophisticatedprocessors of information, enabling quantum computers to function inways that allow them to solve difficult problems that are intractableusing conventional computers. IBM has successfully constructed anddemonstrated the operability of a quantum processor usingsuperconducting qubits (IBM is a registered trademark of InternationalBusiness Machines Corporation in the United States and in othercountries.)

A superconducting qubit includes a Josephson junction. A Josephsonjunction is formed by separating two thin-film superconducting metallayers by a non-superconducting material. When the metal in thesuperconducting layers is caused to become superconducting—e.g. byreducing the temperature of the metal to a specified cryogenictemperature—pairs of electrons can tunnel from one superconducting layerthrough the non-superconducting layer to the other superconductinglayer. In a qubit, the Josephson junction—which functions as adispersive nonlinear inductor—is electrically coupled in parallel withone or more capacitive devices forming a nonlinear microwave oscillator.The oscillator has a resonance/transition frequency determined by thevalue of the inductance and the capacitance in the qubit. Any referenceto the term “qubit” is a reference to a superconducting qubit oscillatorcircuitry that employs a Josephson junction unless expresslydistinguished where used.

The information processed by qubits is carried or transmitted in theform of microwave signals/photons in the range of microwave frequencies.The microwave frequency of a qubit output is determined by the resonancefrequency of the qubit. The microwave signals are captured, processed,and analyzed to decipher the quantum information encoded therein. Areadout circuit is a circuit coupled with the qubit to capture, read,and measure the quantum state of the qubit. An output of the readoutcircuit is information usable by a Q-processor to perform computations.

A superconducting qubit has two quantum states—|0> and |1>. These twostates may be two energy states of atoms, for example, the ground (|0>)and first excited state (|1>) of a superconducting artificial atom(superconducting qubit). Other examples include spin-up and spin-down ofthe nuclear or electronic spins, two positions of a crystalline defect,and two states of a quantum dot. Since the system is of a quantumnature, any combination of the two states is allowed and valid.

For quantum computing using qubits to be reliable, quantum circuits,e.g., the qubits themselves, the readout circuitry associated with thequbits, and other parts of the quantum processor, must not alter theenergy states of the qubit, such as by injecting or dissipating energy,in any significant manner or influence the relative phase between the|0> and |1> states of the qubit. This operational constraint on anycircuit that operates with quantum information necessitates specialconsiderations in fabricating semiconductor and superconductingstructures that are used in such circuits.

The illustrative embodiments recognize that a qubit's resonancefrequency is inherently fixed at the time the qubit is fabricated, i.e.,when the Josephson Junction and the capacitive element of thequbit-oscillator are fabricated on a Q-processor chip. The illustrativeembodiments further recognize that in the simplest implementation of aquantum processor (Q-processor), at least two qubits are needed toimplement a quantum logic gate (Q-gate). Therefore, a Q-processor chipis typically fabricated to have at least 2, but often 8, 16, or morequbits on a single Q-processor chip.

Some qubits are fixed-frequency qubits, i.e., their resonancefrequencies are not changeable. Other qubits are frequency-tunablequbits. A Q-processor can employ fixed-frequency qubits,frequency-tunable qubits, or a combination thereof.

The illustrative embodiments recognize that it is difficult to fabricatesingle-junction transmons or fixed-frequency superconducting qubits withspecific accurate frequencies or accurate frequency differences betweenneighboring qubits. This is mainly because the critical current ofJosephson junctions is not a well-controlled parameter in thefabrication process. This results in a relatively wide-spread in thecritical currents of Josephson junctions having the same design and areaand fabricated on the same chip.

The illustrative embodiments recognize that when the resonancefrequencies of two neighboring coupled qubits on a chip are the same orwithin a threshold band of frequencies or their higher transitionfrequencies are on resonance or close to resonance, then negativeeffects can happen such as, crosstalk, quantum decoherence, energydecay, creation of mixed states, unintended information transfer,quantum state leakage and so on. Having such qubits can also negativelyaffect the performance or utility of certain quantum gates such ascross-resonance gates which have stringent requirements on the spectrumof resonance frequencies of qubits upon which the gate is operating on.Therefore, the illustrative embodiments recognize that one challenge inquantum processors that are based on coupled fixed-frequency qubits isfrequency crowding or frequency collision between adjacent qubits, inparticular, when cross-resonance gates are used.

It is important to note that while the proposed flux-biasing techniqueis motivated by the need to solve frequency collisions of coupled qubitson the same chip which are acted on with cross-resonance gates, theproposed flux-biasing technique is general, and can be applied to otherkinds of quantum devices on chip which require relatively high-densityflux biasing without penetrating the device package.

The illustrative embodiments recognize that a frequency-tunable qubit(hereinafter compactly referred to as a “tunable qubit”) has aflux-dependent inductance, consisting of a superconducting loop thatincludes one or more Josephson junctions. By varying the magnetic fieldthreading the loop, the inductance of the loop changes, which in turnchanges the resonance frequency of the qubit, thus making the qubittunable. The illustrative embodiments recognize that one challenge inquantum processors that are based on tunable-frequency qubits issensitivity to flux noise which leads to dephasing.

Presently, when the frequency of a flux-tunable qubit on a chip has tobe changed, there are two main methods that are used in thestate-of-the-art to apply or change the flux threading the loop of thequbit. The first method is using a global superconducting coil attachedto the qubit-chip package. This method has the advantage of having anexternal fully controllable magnetic source which does not penetrate thedevice package. Such an external source can be filtered well and avoidsseveral negative affects of having magnetic field lines inside of thepackage and near the quantum chip such as, crosstalk, power leakage,noise penetration. The disadvantage of this method is that the qubitscannot be individually controlled and tuned. The second method is usingon-chip magnetic field lines or flux-lines that are placed on adifferent layer or printed circuit board that are near the qubit chip.The advantages of this method are: 1—it is scalable, 2-enableshigh-density flux-line systems for large quantum processors, 3—allowsindividual qubits to be tuned and controlled. The disadvantages of thismethod are: 1—it introduces many possible noise channels between thequantum processor and the external environment which can negativelyaffect the coherence and performance of the quantum processors, 2—it isdifficult to fabricate and route the on-chip flux-lines or magneticfield lines that are near the qubits on different layers or on printedcircuit boards inside the device package.

SUMMARY

The illustrative embodiments provide a superconducting device, a methodand system of fabrication therefore. A superconducting flux-biasingdevice of an embodiment includes a plurality of magnetic flux generatingmembers (member), wherein a first member of the plurality of members isconfigured to magnetically interact with a first qubit in a plurality ofqubits of a quantum processor such that a first magnetic flux of thefirst member causes a first change in a first resonance frequency of thefirst qubit by a first frequency shift value, and each non-correspondingmember of the plurality of members is at a distance from qubitscorresponding to other members of the plurality of members such thatqubits corresponding to other members exhibit less than a thresholdvalue of resonance frequency shift as a result of a magnetic flux of anon-corresponding member.

Another embodiment further includes a structure on which the pluralityof members is fabricated, the structure being distinct from a chip onwhich the quantum processor is fabricated; a fastening mechanism tofasten the structure to a housing of the quantum processor, wherein thefastening mechanism disposes the first member within an influencedistance of the first qubit, wherein the influence distance causes thefirst frequency shift value to be greater than the threshold value.

In another embodiment, the first member is separated from the firstqubit by a first gap, the gap being occupied by partial vacuum.

Another embodiment further includes a first wrapping encasing the firstmember, wherein the first wrapping comprises an electrical insulator.

Another embodiment further includes a wire of a superconductingmaterial, the wire being formed into a coil around a conductive core,the coiled wire and the core together forming the first member, thefirst member being configured to produce the first magnetic flux whensupplied with a direct current.

In another embodiment, the first member produces the first magnetic fluxwhile operating in a range of temperatures between 4 degrees Kelvin and0.0001 degrees Kelvin.

Another embodiment further includes an orientation of the first memberrelative to the first qubit, wherein the orientation comprisespositioning the first member with a cylindrical axis of the first memberbeing orthogonal to a plane of fabrication of the first qubit, with thefirst member is situated below the plane of fabrication.

Another embodiment further includes an orientation of the first memberrelative to the first qubit, wherein the orientation comprisespositioning the first member with a cylindrical axis of the first memberbeing orthogonal to a plane of fabrication of the first qubit, with thefirst member is situated above the plane of fabrication.

Another embodiment further includes an orientation of the first memberrelative to the first qubit, wherein the orientation comprisespositioning the first member with a cylindrical axis of the first memberbeing at a non-orthogonal angle to a plane of fabrication of the firstqubit, with the first member is situated below the plane of fabrication.

Another embodiment further includes a one-to-one correspondence betweenthe first member and the first qubit.

Another embodiment further includes an n-to-one correspondence betweenmore than one members of the plurality of members and the first qubit.

Another embodiment further includes a second member of the plurality ofmembers is configured to magnetically interact with the second qubit inthe plurality of qubits of the quantum processor such that a secondmagnetic flux of the second member causes a second change in a secondresonance frequency of the second qubit by a second frequency shiftvalue, and wherein the second member is at a distance from the firstqubit such that the first qubit exhibits less than the threshold valueof resonance frequency shift as a result of the second magnetic flux.

Another embodiment further includes a pair of connectors coupled to thefirst member, wherein changing a value of a direct current passingthrough the pair of connectors causes a corresponding change in thefirst frequency shift value of the first qubit.

Another embodiment further includes a first number of coil turns in thefirst member; a second number of coil turns in a second member of theplurality of members, wherein the second member causes a secondfrequency shift value in a second qubit corresponding to the secondmember as compared to the first frequency shift value in the first qubitwhen a first value of direct current is passed through the first memberand the second member.

In another embodiment, the plurality of members is positioned outside ahousing that contains the plurality of qubits, and wherein the pluralityof members are removable via one cover of the housing.

In another embodiment, the plurality of members is thermalized to adilution fridge stage through a housing that contains the plurality ofqubits.

In another embodiment, a cylindrical axis of each member is aligned witha center of a resonance loop of a corresponding qubit from the pluralityof qubits.

Another embodiment further includes a filtering circuit fabricated inconjunction with the plurality of members, wherein the filtering circuitfilters an electrical effect of the plurality of members.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristics of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objectives, and advantages thereof,will best be understood by reference to the following detaileddescription of the illustrative embodiments when read in conjunctionwith the accompanying drawings, wherein:

FIG. 1 depicts a block diagram of an example configuration of aprior-art of a global superconducting magnetic coil which can beimproved in accordance with an illustrative embodiment;

FIG. 2 depicts a block diagram of an example configuration of anapparatus for flux biasing in superconducting quantum processors inaccordance with an illustrative embodiment;

FIG. 3 depicts an example implementation of L-Q pairs for flux biasingin superconducting quantum processors in accordance with an illustrativeembodiment;

FIG. 4 depicts another view of an example implementation of L-Q pairsfor flux biasing in superconducting quantum processors in accordancewith an illustrative embodiment;

FIG. 5 depicts another view of an example implementation of L-Q pairsfor flux biasing in superconducting quantum processors in accordancewith an illustrative embodiment;

FIG. 6 depicts another view of an example implementation of L-Q pairsfor flux biasing in superconducting quantum processors in accordancewith an illustrative embodiment;

FIG. 7 depicts another view of an example implementation of L-Q pairsfor flux biasing in superconducting quantum processors in accordancewith an illustrative embodiment;

FIG. 8 depicts another view of an example implementation of L-Q pairsfor flux biasing in superconducting quantum processors in accordancewith an illustrative embodiment;

FIG. 9 depicts another view of an example implementation of L-Q pairsfor flux biasing in superconducting quantum processors in accordancewith an illustrative embodiment;

FIG. 10 depicts a view of an example connection between inductivemembers to multi-layer substrate or printed circuit board;

FIG. 11 depicts an example relative orientation of a coil and a qubit inan L-Q pair in accordance with an illustrative embodiment;

FIG. 12 depicts another example relative orientation of a coil and aqubit in an L-Q pair in accordance with an illustrative embodiment;

FIG. 13 depicts another example relative orientation of a coil and aqubit in an L-Q pair in accordance with an illustrative embodiment;

FIG. 14 depicts another example relative orientation of a coil and aqubit in an L-Q pair in accordance with an illustrative embodiment;

FIG. 15 depicts a flowchart of an example process of fabricating anapparatus for flux biasing in superconducting quantum processors inaccordance with an illustrative embodiment;

FIG. 16 depicts a flowchart of an example process of fabricating anapparatus for static flux biasing in superconducting quantum processorsin accordance with an illustrative embodiment; and

FIG. 17 depicts a flowchart of an example process of fabricating anapparatus for dynamic flux biasing in superconducting quantum processorsin accordance with an illustrative embodiment.

DETAILED DESCRIPTION

The illustrative embodiments used to describe the invention generallyaddress and solve the above-described needs for individually tunablequbits on a single chip. The illustrative embodiments provide a methodand apparatus for flux biasing in superconducting quantum processors.

An operation described herein as occurring with respect to a frequencyor frequencies should be interpreted as occurring with respect to asignal of that frequency or frequencies. All references to a “signal”are references to a microwave signal unless expressly distinguishedwhere used.

An embodiment provides a configuration of an apparatus for flux biasingin superconducting quantum processors. Another embodiment provides afabrication method for the apparatus for flux biasing in superconductingquantum processors, such that the method can be implemented as asoftware application. The application implementing a fabrication methodembodiment can be configured to operate in conjunction with an existingsuperconductor fabrication system—such as a lithography system.

For the clarity of the description, and without implying any limitationthereto, the illustrative embodiments are described using some exampleconfigurations. From this disclosure, those of ordinary skill in the artwill be able to conceive many alterations, adaptations, andmodifications of a described configuration for achieving a describedpurpose, and the same are contemplated within the scope of theillustrative embodiments.

Furthermore, simplified diagrams of the example qubits, magnetic coilsor magnetic flux inducing structures, housing, casing, and other circuitcomponents are used in the figures and the illustrative embodiments. Inan actual fabrication or circuit, additional structures or componentthat are not shown or described herein, or structures or componentsdifferent from those shown but for the purpose described herein may bepresent without departing the scope of the illustrative embodiments.

Furthermore, the illustrative embodiments are described with respect tospecific actual or hypothetical components only as examples. The stepsdescribed by the various illustrative embodiments can be adapted forfabricating a circuit using a variety of components that can be purposedor repurposed to provide a function in a described manner, and suchadaptations are contemplated within the scope of the illustrativeembodiments.

The illustrative embodiments are described with respect to certain typesof materials, electrical properties, steps, shapes, sizes, numerosity,frequencies, circuits, components, and applications only as examples.Any specific manifestations of these and other similar artifacts are notintended to be limiting to the invention. Any suitable manifestation ofthese and other similar artifacts can be selected within the scope ofthe illustrative embodiments.

The examples in this disclosure are used only for the clarity of thedescription and are not limiting to the illustrative embodiments. Anyadvantages listed herein are only examples and are not intended to belimiting to the illustrative embodiments. Additional or differentadvantages may be realized by specific illustrative embodiments.Furthermore, a particular illustrative embodiment may have some, all, ornone of the advantages listed above.

With reference to FIG. 1, this figure depicts a block diagram of anexample configuration of a prior-art of a global superconductingmagnetic coil which can be improved in accordance with an illustrativeembodiment. Printed circuit board (PCB) 100 includes microwaveconnectors 101, chip 102, and other components as may be needed in animplementation. Chip 102 is an example of a Q-processor comprising aplurality of qubits, e.g., qubits 104 and 106. In one embodiment, chip102 can be mounted on printed circuit board 100 using a housing, anon-limiting example of which is described herein.

Global superconducting magnetic coil 108 is a flux inducing structurethat is placed near chip 102 to provide biasing flux to all qubits onchip 102. Global superconducting magnetic coil 108 is distinct andseparate from chip 102. Global superconducting magnetic coil 108 isformed by winding a superconducting wire with very thin insulatingcoating around a metallic core or rod, the two ends of thesuperconducting wire terminating at contacts 110 and 112. ElectricDirect current (DC)—current A—flows through the coil to generatemagnetic flux ϕ. Flux ϕ change the output frequencies of qubits 104 and106 by some amount. The flux dependence of the superconducting loop isperiodic. The amount of flux that threads the qubit loops depend ontheir distances from global superconducting magnetic coil 108 and alsoon the background magnetic field which might be unequal for thedifferent qubits. Generally, once the position of global superconductingmagnetic coil 108 is fixed relative to chip 102, and the magneticenvironment of chip 102 is stabilized in an installation, the changes inthe frequencies of qubits 104 and 106 on chip 102 cannot be tunedindependently of each other using a global superconducting magnetic coil108.

With reference to FIG. 2, this figure depicts a block diagram of anexample configuration of an apparatus for flux biasing insuperconducting quantum processors in accordance with an illustrativeembodiment. Chip 102 including qubits 104 and 106 is reused from FIG. 1.

In contrast with global superconducting magnetic coil 108 of FIG. 1,configuration 200 of FIG. 2 depicts a separate magnetic coil for anindividual qubit on chip 102. For example, coil 204 flux biases qubit104 (and therefore is associated with qubit 104) and coil 206 fluxbiases qubit 106 (and therefore is associated with qubit 106).Hereinafter, the pair of a qubit-specific coil and the correspondingqubit is referred to as an “L-Q pair”. An embodiment forms and positionssuch a qubit-specific coil relative to the corresponding qubit in such amanner that the magnetic field lines from the qubit-specific coilinteract mainly with the corresponding qubit and any magneticinterference with adjacent qubits is maintained within an acceptabletolerance limit.

Generally, a magnetic coil generates magnetic flux, which passthrough—or threads—the superconducting loop of a qubit—which includesthe inductance of a Josephson junction. The flux threading through theloop of the qubit causes a change in the inductance of the Josephsonjunction, which in turn results in a change in the resonance frequencyof the qubit loop. Operating in this manner, coil 204 only interactswith qubit 104 in a manner to cause a substantial (greater than thethreshold) amount of change or shift in qubit 104's frequency. Coil 206operates relative to qubit 106 in a similar manner. Coil 204 preferablydoes not cause any shift in qubit 106's frequency, or any such shift isnegligible compared to the frequency spectrum of qubits in quantumprocessors. Coil 206 behaves in a similar manner relative to qubit 104.

Furthermore, each qubit-specific coil is optionally mounted on platform202, which is a separate removable platform, e.g., a separate PCB.Platform 202 is usable to position each qubit-specific coil relative tochip 102 in a movable manner, removable manner, or both. For example, inone embodiment, one qubit-specific coil may be moved or repositionedrelative to a corresponding qubit, e.g., for improving the magneticinteraction with a corresponding qubit, reducing undesirableinterference with a non-corresponding qubit, or some combination ofthese and other objectives.

Change in the magnitude of the magnetic field threading thesuperconducting Josephson loop changes the flux threading that loop. Forexample, in the depicted non-limiting example orientation, coil 204 maybe moved or repositioned away from qubit 106 while still keeping qubit104 within the magnetic field lines of flux ϕ1. Similarly, coil 206 maybe moved or repositioned away from qubit 104 while still keeping qubit106 within the magnetic field lines of flux ϕ2.

In another example embodiment, in the depicted non-limiting exampleorientation, coil 204 may be moved closer to qubit 104 along coil 204'scylindrical axis to reduce a gap between coil 204 and qubit 104,resulting in a stronger magnetic coupling between coil 204 and qubit104. Conversely, coil 206 may be moved farther from qubit 106 along coil206's cylindrical axis to increase a gap between coil 206 and qubit 106,resulting in a weaker magnetic coupling between coil 206 and qubit 106.

The depicted orientation of FIG. 2, which shows the qubit-specific coils204-206 positioned below qubits 104-106, respectively, is a preferredorientation but is not intended to be limiting. As will become apparentfrom this disclosure, a qubit-specific coil can be oriented relative tothe corresponding qubit in other orientations as well to achieve thequbit-specific frequency shifting effect on corresponding qubits.Different orientations yield different amounts of fluxes ranging fromsignificant to negligible. While some orientations might be useful inpresently available superconducting q-processor implementations, otherorientations might find utility in other potential quantum devices whichemploy non-superconducting qubits with on-chip planar loops.

Furthermore, in one embodiment, the magnetic flux of each qubit-specificcoil is independently and dynamically controlled by adjusting thecurrent supplied to the qubit-specific coil through a dedicated pair ofcontacts for that qubit-specific coil. For example, current A1 suppliedthrough contacts 208-210 generates from coil 204 a magnetic field whichproduces flux ϕ1 threading through the loop of qubit 104 and causes achange F1 in qubit 104's resonance frequency. Changing the current toA1′ (not shown) will consequently generate flux ϕ1′ (not shown) fromcoil 204 and cause a different change F1′ in qubit 104's resonancefrequency. Qubit-specific coil 206 operates in a similar manner. currentA2 supplied through contacts 212-214 generates flux ϕ2 from coil 206 andcauses a change F2 in qubit 106's resonance frequency. Changing thecurrent to A2′ (not shown) will consequently generate flux ϕ2′ (notshown) from coil 206 and cause a different change F2′ in qubit 106'sresonance frequency.

Without implying that any particular embodiment provides any specificadvantage or property, some of the advantages or properties that may berealized from implementing an embodiment in a specific manner includebut are not limited to: 1—the superconducting coils are external to thepackage; 2—each coil primarily flux-biases one qubit; 3—these coils areeasy to thermalize within the device package and through the mountingstructure; 4—these coils can be small size, if the coils are locatedvery close to the qubits, e.g., the package has a thin wall on the coilsside; 5—these coils are mounted into precise slots-pattern within thedevice package whose centers are aligned with the qubits center; 6—thecoils are soldered or bonded to superconducting printed circuit board ormulti-layer substrate; 7—the printed circuit board could consist ofmultiple superconducting layers carrying current to different coilswithout overlaps and with little interference; and 8—the coils can befiltered using filters integrated into the printed circuit board or themulti-layer substrate.

Different quantum processing applications can have differentrequirements for flux-biasing the qubits. In some implementations,magnetic field biasing may have to be applied perpendicularly to theplane of superconducting qubits. In some other implementations, magneticfield biasing may have to be applied in parallel to the plane ofsuperconducting qubits or other quantum devices. Such other requirementsand implementations are contemplated within the scope of theillustrative embodiments. The coils in L-Q pairs of an embodiment can beoriented differently relative to their corresponding qubits.Furthermore, in one embodiment, a coil can have a one-to-onecorrespondence with a qubit; in another embodiment, a coil can have aone-to-n correspondence with a plurality of qubits; in anotherembodiment, n coils can have an n-to one correspondence with a qubitwhere several coils correspond to a single qubit; in another embodiment,n coils can have an n-to-m correspondence with qubits where a set of ncoils correspond to a set of m qubits. In another embodiment, a coil canhave a zero-to-one correspondence with a qubit where no coil correspondsto certain qubits on a chip.

As can be seen from the variety of configurations disclosed, each qubitcan be independently controlled for resonance frequency shift.Furthermore, the shift can be statically set or dynamically changed foran individual qubit on a multi-qubit chip. Additionally, qubits andcoils can be oriented differently relative to one another to achieve theshifts, giving a variety of implementation alternatives inspace-constrained implementations. Malfunction or maloperation in oneL-Q pair can be easily identified and remedied without disturbing otherL-Q pairs. All these advantages, and other features are further usefulbecause the set of coils according to an embodiment can be implementedoff-chip, i.e., not on the qubit chip itself. Some off-chipimplementations are now described as non-limiting exampleimplementations in FIGS. 3-10.

With reference to FIG. 3, this figure depicts an example implementationof L-Q pairs for flux biasing in superconducting quantum processors inaccordance with an illustrative embodiment. Configuration 300 depicts apresently used implementation of cryogenic q-processors. PCB 302 formsthe structure onto which a housing for a Q-processor is mounted. In someimplementations, the housing is fabricated from Copper due to the goodelectrical conductivity and good thermalization properties of the metalat cryogenic temperatures. The housing comprises top cover 304 andbottom cover 306, which together enclose a volume in which theQ-processor chip (not shown) is installed.

A set of connectors 308 is also configured on PCB 302 to couple theassembly with external circuits. Connectors 308 can be electricalcouplings, microwave couplings, thermal couplings, mechanical couplings,or some combination of these and other types of couplings used inQ-processor implementations.

With reference to FIG. 4, this figure depicts another view of an exampleimplementation of L-Q pairs for flux biasing in superconducting quantumprocessors in accordance with an illustrative embodiment. Another viewof configuration 300 shows some example connectors 308 in the form ofmicrowave connectors 402. A detachable platform structure that can bemounted on or through bottom cover 306 carries the coils of L-Q pairs(not visible) on the side that faces the volume inside the housing.Detachable platform structure 404 can be implemented as a second PCB.Contacts for each coil carried on structure 404 can be placed (notshown) on structure 404 (not shown). Platform structure 404 can bedetachably fastened to the housing using suitable fasteners 406, e.g.,screws (not shown), which fasten platform structure 404 to bottom cover306.

With reference to FIG. 5, this figure depicts another view of an exampleimplementation of L-Q pairs for flux biasing in superconducting quantumprocessors in accordance with an illustrative embodiment. Another viewof configuration 300 shows fasteners 502 to attach top cover 304 tobottom cover 306. Fasteners 406 attach platform structure 404 to bottomcover 306.

Individual coils of L-Q pairs correspond to qubits on Q-processor chip504. Some qubit locations 506 are visible in the depicted view. Somecoils 508 that individually correspond to qubits in locations 506 arealso visible. In one embodiment, each coils 508 is a superconductingmagnetic coil, configured to operate at the same or similar temperatureas the corresponding qubit.

A wrapped coil is placed in sufficiently close proximity of thecorresponding qubit such that the magnetic flux of the coil can interactwith and alter the frequency of the qubit's resonance frequency. In oneembodiment, as depicted, a wrapped coil 508 is positioned below thecorresponding qubit location 506. Coils 508 are attached to detachableplatform structure 404, qubits 506 are fabricated on chip 504, and thecoils and the qubits of L-Q pairs interact across a medium, e.g.,practically attainable vacuum, high-conductivity copper, silicon, asdescribed herein. The conductor of a coil is terminated at a pair ofcontacts on or through platform structure 404. The pair of contacts ofeach coil can be used to flow the current through the individual coil.

With reference to FIG. 6, this figure depicts another view of an exampleimplementation of L-Q pairs for flux biasing in superconducting quantumprocessors in accordance with an illustrative embodiment. Another viewof configuration 300 shows a non-limiting example detailed dispositionof a coil 508 relative to a corresponding qubit 506.

With reference to FIG. 7, this figure depicts another view of an exampleimplementation of L-Q pairs for flux biasing in superconducting quantumprocessors in accordance with an illustrative embodiment. Another viewof configuration 300 shows another non-limiting example detaileddisposition of a coil 508 relative to a corresponding qubit 506 insidethe housing.

With reference to FIG. 8, this figure depicts another view of an exampleimplementation of L-Q pairs for flux biasing in superconducting quantumprocessors in accordance with an illustrative embodiment. Another viewof configuration 300 shows another non-limiting example detaileddisposition of a coil 508 relative to a corresponding qubit 506 insidethe housing. In one embodiment, coil 508 is formed by wrapping a thinlyinsulated superconducting wire around a conductive metal core, e.g., anoxygen-free high conductivity copper rod. An insulator wrapping aroundcoil 508 is optional.

With reference to FIG. 9, this figure depicts another view of an exampleimplementation of L-Q pairs for flux biasing in superconducting quantumprocessors in accordance with an illustrative embodiment. This view ofconfiguration 300 shows an example non-limiting method of fasteningplatform structure 404, top cover 304 of FIG. 3, and bottom cover 306 ofFIG. 3 together. Coils 508 of FIG. 5 are also visible in this figure.

With reference to FIG. 10, this figure depicts another view of anexample implementation of L-Q pairs for flux biasing in superconductingquantum processors in accordance with an illustrative embodiment. Thisview of configuration 300 shows an example non-limiting configuration ofan example coil 508. Coils 508 are attached, affixed, or otherwisemounted on platform structure 404. Ends 1002 and 1004 of thesuperconducting wire of each coil 508 are suitably connected, bonded,wirebonded, or exposed to connect with contacts (not shown), which maybe placed on the opposite side of platform structure 404.

With reference to FIG. 11, this figure depicts an example relativeorientation of a coil and a qubit in an L-Q pair in accordance with anillustrative embodiment. Configuration 1100 depicts a simplified blockdiagram of an example L-Q pair. In this configuration, qubit 1102 andcoil 1104 are disposed relative to each other in a manner shown in FIGS.3-10. Specifically, in this example orientation, coil 1104 is mountedunder qubit 1102 and projects magnetic field lines up towards qubit1102.

With reference to FIG. 12, this figure depicts another example relativeorientation of a coil and a qubit in an L-Q pair in accordance with anillustrative embodiment. Configuration 1200 depicts a simplified blockdiagram of an example L-Q pair. In this configuration, coil 1204 ismounted above qubit 1202 and projects magnetic field lines down towardsqubit 1202.

With reference to FIG. 13, this figure depicts another example relativeorientation of a coil and a qubit in an L-Q pair in accordance with anillustrative embodiment. Configuration 1300 depicts a simplified blockdiagram of an example L-Q pair. In this configuration, coil 1304 ismounted substantially in parallel to qubit 1302—without contacting qubit1302. Coil 1304 projects magnetic field lines substantially parallellyto the qubit 1302.

With reference to FIG. 14, this figure depicts another example relativeorientation of a coil and a qubit in an L-Q pair in accordance with anillustrative embodiment. Configuration 1400 depicts a simplified blockdiagram of an example L-Q pair. In this configuration, coil 1404 ismounted at an angle relative to qubit 1402—without contacting qubit1402. Coil 1404 projects magnetic field lines at an angle towards qubit1402.

The orientations of FIGS. 13-14 or a similar orientation may not bedesirable in the circumstance exemplified herein, but may be desirableunder different circumstances. For example, if accessibility to a qubitpackage was to be obstructed in an implementation, and access wasreadily available in the qubit housing from a direction depicted in thisfigure, such an orientation could be used for flux biasing. Theseorientations may cause a greater interference with neighboring qubits insome cases, and may require special tooling or technique to fabricate.

With reference to FIG. 15, this figure depicts a flowchart of an exampleprocess of fabricating an apparatus for flux biasing in superconductingquantum processors in accordance with an illustrative embodiment.Process 1500 can be implemented in a fabrication system, e.g., in asoftware application that operates the fabrication system, to cause thedescribed operations.

The embodiment configures an inductive element (L) relative to a qubit(Q) in a q-processor (block 1502). The embodiment configures a pluralityof inductive elements—the qubit-specific coils described herein—relativeto as many qubits whose resonance frequencies may be needed to becontrolled in a given Q-processor chip.

For a given L-Q pair, the embodiment configures the inductive element Lto produce a specific magnetic flux value ϕ to cause a shift S in themicrowave resonance frequency of the corresponding qubit Q (block 1504).The embodiment repeats block 1504 as many times as may be needed toconfigure various L-Q pairs in a given implementation. The embodimentends process 1500 thereafter.

With reference to FIG. 16, this figure depicts a flowchart of an exampleprocess of fabricating an apparatus for static flux biasing insuperconducting quantum processors in accordance with an illustrativeembodiment. Process 1600 can be implemented as block 1504 in FIG. 15.

When a need exists for static shift in a qubit's resonance frequency,the embodiment configured an inductive element (L, qubit-specific coil)corresponding to that qubit with a predetermined number of coil turns,which would produce the desired value of magnetic flux at apredetermined electrical current value (DC), the magnetic flux causingthe desired static shift in the resonance frequency of the qubit (block1602). Different inductive elements can be statically configured tocause different static shifts in their respective qubits in a similarmanner. The embodiment ends process 1600 thereafter.

With reference to FIG. 17, this figure depicts a flowchart of an exampleprocess of fabricating an apparatus for dynamic flux biasing insuperconducting quantum processors in accordance with an illustrativeembodiment. Process 1700 can be implemented as block 1504 in FIG. 15.

When a need exists for dynamically adjustable shift in a qubit'sresonance frequency, the embodiment configured an inductive element (L,qubit-specific coil) corresponding to that qubit with individualconnectors, which can receive dynamically adjustable electrical currentvalue (DC), which cause an adjustable magnetic flux, the adjustablemagnetic flux causing the desired adjustable shift in the resonancefrequency of the qubit (block 1702). Different inductive elements can bedynamically configured to receive different currents causing differentadjustable shifts in their respective qubits in a similar manner. Theembodiment ends process 1700 thereafter.

Within the scope of the illustrative embodiments, it is contemplatedthat the amount of flux/magnetic field that the each coil can generatewithin the qubit loop, depend in general on—1) the number of turns ofthe superconducting wire wound around the core—the larger the number thelarger the induced field; 2) the applied DC current through the wire—thelarger the current, the larger the field; and 3) the distance betweenthe near end of the coil and the qubit loop—the smaller the distance thelarger the field.

Combinations of static and dynamic arrangements are contemplated withinthe scope of the illustrative embodiments. For example, different coilsof different numbers of turns can receive variable current to causedifferent flux changes at different qubits. Such combinations andeffects can be implemented without departing the scope of theillustrative embodiments.

The circuit elements of the flux-biasing apparatus and connectionsthereto can be made of superconducting material. Examples ofsuperconducting materials (at low temperatures, such as about 10-100millikelvin (mK), or about 4 K) include Niobium, Aluminum, Tantalum,etc. For example, the Josephson junctions are made of superconductingmaterial, and their tunnel junctions can be made of a thin tunnelbarrier, such as an aluminum oxide. The capacitors can be made ofsuperconducting material separated by low-loss dielectric material. Thetransmission lines (i.e., wires) connecting the various elements can bemade of a superconducting material.

Various embodiments of the present invention are described herein withreference to the related drawings. Alternative embodiments can bedevised without departing from the scope of this invention. Althoughvarious connections and positional relationships (e.g., over, below,adjacent, etc.) are set forth between elements in the followingdescription and in the drawings, persons skilled in the art willrecognize that many of the positional relationships described herein areorientation-independent when the described functionality is maintainedeven though the orientation is changed. These connections and/orpositional relationships, unless specified otherwise, can be direct orindirect, and the present invention is not intended to be limiting inthis respect. Accordingly, a coupling of entities can refer to either adirect or an indirect coupling, and a positional relationship betweenentities can be a direct or indirect positional relationship. As anexample of an indirect positional relationship, references in thepresent description to forming layer “A” over layer “B” includesituations in which one or more intermediate layers (e.g., layer “C”) isbetween layer “A” and layer “B” as long as the relevant characteristicsand functionalities of layer “A” and layer “B” are not substantiallychanged by the intermediate layer(s).

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

Additionally, the term “illustrative” is used herein to mean “serving asan example, instance or illustration.” Any embodiment or designdescribed herein as “illustrative” is not necessarily to be construed aspreferred or advantageous over other embodiments or designs. The terms“at least one” and “one or more” are understood to include any integernumber greater than or equal to one, i.e. one, two, three, four, etc.The terms “a plurality” are understood to include any integer numbergreater than or equal to two, i.e. two, three, four, five, etc. The term“connection” can include an indirect “connection” and a direct“connection.”

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedcan include a particular feature, structure, or characteristic, butevery embodiment may or may not include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

The terms “about,” “substantially,” “approximately,” and variationsthereof, are intended to include the degree of error associated withmeasurement of the particular quantity based upon the equipmentavailable at the time of filing the application. For example, “about”can include a range of ±8% or 5%, or 2% of a given value.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdescribed herein.

What is claimed is:
 1. A method to form a flux-biasing device, themethod comprising: forming a plurality of magnetic flux generatingmembers (member); forming a detachable platform located external to apackage and a housing that contains the package, the package comprisinga plurality of qubits of a quantum processor, wherein the plurality ofmembers are installed on the detachable platform; configuring a firstmember of the plurality of members to magnetically interact with a firstqubit in the plurality of qubits of the quantum processor such that afirst magnetic flux of the first member causes a first change in a firstresonance frequency of the first qubit by a first frequency shift value;and distancing each non-corresponding member of the plurality of membersat least at a threshold distance from qubits corresponding to othermembers of the plurality of members such that qubits corresponding toother members exhibit less than a threshold value of resonance frequencyshift as a result of a magnetic flux of a non-corresponding member. 2.The method of claim 1, further comprising: encasing the package in ahousing; detachably fastening, using a fastening mechanism, thedetachable platform to a removable cover of the housing, wherein thefastening mechanism disposes the first member within an influencedistance of the first qubit, wherein the influence distance causes thefirst frequency shift value to be greater than the threshold value. 3.The method of claim 1, wherein the first member is separated from thefirst qubit by a first gap, the gap being occupied by at least one froma group comprising (i) partial vacuum and (ii) a solid.
 4. The method ofclaim 1, further comprising: forming a coil around a conductive coreusing a wire of a superconducting material, the coil and the coretogether forming the first member, the first member being configured toproduce the first magnetic flux when supplied with a direct current. 5.The method of claim 4, further comprising: covering the wire using aninsulating layer; and forming a metallic contact, wherein the contactforms an electrical connection between the wire and at least one from agroup comprising (i) a layer of the detachable platform and (ii) asubstrate.
 6. The method of claim 4, wherein the first member producesthe first magnetic flux while operating in a range of temperaturesbetween 4 degrees Kelvin and 0.0001 degrees Kelvin.
 7. The method ofclaim 1, further comprising: orientating the first member relative tothe first qubit, wherein the orientating comprises positioning the firstmember with a cylindrical axis of the first member being orthogonal to aplane of fabrication of the first qubit, with the first member issituated below the plane of fabrication.
 8. The method of claim 1,further comprising: orientating the first member relative to the firstqubit, wherein the orientating comprises positioning the first memberwith a cylindrical axis of the first member being orthogonal to a planeof fabrication of the first qubit, with the first member is situatedabove the plane of fabrication.
 9. The method of claim 1, furthercomprising: orientating the first member relative to the first qubit,wherein the orientating comprises positioning the first member with acylindrical axis of the first member being at a non-orthogonal angle toa plane of fabrication of the first qubit, with the first member issituated below the plane of fabrication.
 10. The method of claim 1,further comprising: configuring a one-to-one correspondence between thefirst member and the first qubit.
 11. The method of claim 1, furthercomprising: configuring an n-to-one correspondence between more than onemembers of the plurality of members and the first qubit.
 12. The methodof claim 1, further comprising: configuring a second member of theplurality of members to magnetically interact with a second qubit in theplurality of qubits of the quantum processor such that a second magneticflux of the second member causes a second change in a second resonancefrequency of the second qubit by a second frequency shift value, andwherein locating the second member at such a distance from the firstqubit such that the first resonance frequency of the first qubit remainsstable within a tolerance regardless of the second member.
 13. Themethod of claim 1, further comprising: coupling a pair of connectors tothe first member, wherein changing a value of a direct current passingthrough the pair of connectors causes a corresponding change in thefirst frequency shift value of the first qubit.
 14. The method of claim1, further comprising: configuring a first number of coil turns in thefirst member; and configuring a second number of coil turns in a secondmember of the plurality of members.
 15. The method of claim 1, whereinthe plurality of members is positioned outside a housing that containsthe plurality of qubits, and wherein the plurality of members areremovable via one cover of the housing.
 16. The method of claim 1,wherein the plurality of members is thermalized to a dilution fridgestage through a housing that contains the plurality of qubits.
 17. Themethod of claim 1, wherein a cylindrical axis of each member is alignedwith a center of a superconducting loop of a corresponding qubit fromthe plurality of qubits.
 18. The method of claim 1, further comprising:fabricating a filtering circuit in conjunction with the plurality ofmembers, wherein the filtering circuit filters an electrical noisecarried by the plurality of members.
 19. A method to form a flux-biasingdevice, the method comprising: forming a plurality of magnetic fluxgenerating members (member); configuring a first member of the pluralityof members to magnetically interact with a first qubit in a plurality ofqubits of a quantum processor such that a first magnetic flux of thefirst member causes a first change in a first resonance frequency of thefirst qubit by a first frequency shift value; and distancing eachnon-corresponding member of the plurality of members from qubitscorresponding to other members of the plurality of members such thatqubits corresponding to other members exhibit less than a thresholdvalue of resonance frequency shift as a result of a magnetic flux of anon-corresponding member.
 20. A superconductor fabrication system whichwhen operated to fabricate a flux-biasing device performs operationscomprising: forming a plurality of magnetic flux generating members(member); configuring a first member of the plurality of members tomagnetically interact with a first qubit in a plurality of qubits of aquantum processor such that a first magnetic flux of the first membercauses a first change in a first resonance frequency of the first qubitby a first frequency shift value; and distancing each non-correspondingmember of the plurality of members from qubits corresponding to othermembers of the plurality of members such that qubits corresponding toother members exhibit less than a threshold value of resonance frequencyshift as a result of a magnetic flux of a non-corresponding member.